11:30 AM - 11:45 AM
[R6-06] Chronological re-examination of the Kanmon Group, Southwest Japan: and remaining unsolved problems
Keywords:Kanmon Group, detrital, zircon age, sandstone, tuff
Stratigraphic studies were capable in fore arc sediments because they consist mainly of marine sediments which contain index fossils and radiolarian fossils. Whereas intra- and back-arc sediments which consist of terrestrial sediments had no other choice than to correlate fresh-water fauna and/or apply radiometric ages which have low reliability. After decades of developments, zircon U-Pb age method expand application. It is applied not only to clarify eruption/deposition age of tuff beds but also age limitation and provenance studies of clastic rocks.
The Kanmon Group is the one of the intra-arc sediments. Although detrital zircon dating is also applied, it is not statistically significant because of too small number of analyses to interpret. To re-examination of the deposition ages and provenances of the Kanmon Group, we analyzed detrital zircon ages. Age results of this study were shown in the Table and Fig. (A).
New obtained YC1σ of the Sujigahama and Shihama formations indicate 97 Ma and 101 Ma, respectively. Moreover, considering the age of the Kawara granite (94.8 ± 0.9 Ma), the age of the upper most of the Shimonoseki Subgroup is thought to be in Cenomanian. On the other hand, YC1σ of the Sengoku Formation, the lowermost of the Wakino Subgroup and the Yoshimo Formation, Toyonishi Group indicate similar values of 128 Ma and 130 Ma, respectively. The fact exhibits that the formations are contemporaneous heterotopic facies each other.
Contradictions about ages are also shown. “The age of the tuff bet” in the Kitahikoshima Formation was reported as 106.3 ± 1.0 Ma which is older than YC1σ of the Shiohama Formation which is stratigraphically lower than the Kitahikoshima Formation. The contradiction can be solved to think that the zircon grains were detrital origin and which YC1σ indicate 99.3 ± 1.3 Ma. Another contradiction is that the Lower Wakamiya Formation is too young. Additional research is necessary to solve the contradiction.
The Kanmon Group is the one of the intra-arc sediments. Although detrital zircon dating is also applied, it is not statistically significant because of too small number of analyses to interpret. To re-examination of the deposition ages and provenances of the Kanmon Group, we analyzed detrital zircon ages. Age results of this study were shown in the Table and Fig. (A).
New obtained YC1σ of the Sujigahama and Shihama formations indicate 97 Ma and 101 Ma, respectively. Moreover, considering the age of the Kawara granite (94.8 ± 0.9 Ma), the age of the upper most of the Shimonoseki Subgroup is thought to be in Cenomanian. On the other hand, YC1σ of the Sengoku Formation, the lowermost of the Wakino Subgroup and the Yoshimo Formation, Toyonishi Group indicate similar values of 128 Ma and 130 Ma, respectively. The fact exhibits that the formations are contemporaneous heterotopic facies each other.
Contradictions about ages are also shown. “The age of the tuff bet” in the Kitahikoshima Formation was reported as 106.3 ± 1.0 Ma which is older than YC1σ of the Shiohama Formation which is stratigraphically lower than the Kitahikoshima Formation. The contradiction can be solved to think that the zircon grains were detrital origin and which YC1σ indicate 99.3 ± 1.3 Ma. Another contradiction is that the Lower Wakamiya Formation is too young. Additional research is necessary to solve the contradiction.